The present invention relates to methods of forming semiconductor substrates and substrates formed thereby, and more particularly to methods of forming compound semiconductor substrates and compound semiconductor substrates formed thereby.
Compound semiconductor materials such as gallium nitride (GaN) have been widely investigated as suitable substrate materials for microelectronic devices including but not limited to transistors, field emitters and optoelectronic devices. It will be understood that, as used herein, compound semiconductor materials may include III-V and II-VI alloys, for example. Reference to specific compound semiconductors such as gallium nitride will also be understood to include a family of gallium nitride alloys such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride.
A major problem in fabricating gallium nitride-based microelectronic devices is the fabrication of gallium nitride semiconductor layers having low defect densities. It is known that one contributor to defect density is lattice mismatch with the substrate on which the gallium nitride layer is grown. Thus, although gallium nitride layers have been grown on sapphire substrates, it is known to reduce defect density by growing gallium nitride layers on aluminum nitride buffer layers which are themselves formed on silicon carbide substrates. Notwithstanding these advances, continued reduction in defect density is desirable. It also is known to produce low defect density gallium nitride layers by forming a mask on a layer of gallium nitride, the mask including at least one opening that exposes the underlying layer of gallium nitride, and laterally growing the underlying layer of gallium nitride through the at least one opening and onto the mask. This technique often is referred to as xe2x80x9cEpitaxial Lateral Overgrowthxe2x80x9d (ELO). The layer of gallium nitride may be laterally grown until the gallium nitride coalesces on the mask to form a single layer on the mask. In order to form a continuous layer of gallium nitride with relatively low defect density, a second mask may be formed on the laterally overgrown gallium nitride layer, that includes at least one opening that is offset from the underlying mask. ELO then again is performed through the openings in the second mask to thereby overgrow a second low defect density continuous gallium nitride layer. Microelectronic devices then may be formed in this second overgrown layer. ELO of gallium nitride is described, for example, in the publications entitled Lateral Epitaxy of Low Defect Density GaN Layers Via Organometallic Vapor Phase Epitaxy to Nam et al., Appl. Phys. Lett. Vol. 71, No. 18, Nov. 3, 1997, pp. 2638-2640; and Dislocation Density Reduction Via Lateral Epitaxy in Selectively Grown GaN Structures to Zheleva et al, Appl. Phys. Lett., Vol. 71, No. 17, Oct. 7, 1997, pp. 2472-2474, the disclosures of which are hereby incorporated herein by reference.
It also is known to produce a layer of gallium nitride with low defect density by forming at least one trench or post in an underlying layer of gallium nitride to define at least one sidewall therein. A layer of gallium nitride is then laterally grown from the at least one sidewall which acts as a xe2x80x9cseedxe2x80x9d. Lateral growth preferably takes place until the laterally grown layers coalesce within the trenches. Lateral growth also preferably continues until the gallium nitride layer that is grown from the sidewalls laterally overgrows onto the tops of the posts. In order to facilitate lateral growth and produce nucleation of gallium nitride and growth in the vertical direction, the top of the posts and/or the trench floors may be masked.
Lateral growth from the sidewalls of trenches and/or posts also is referred to as xe2x80x9cpendeoepitaxyxe2x80x9d and is described, for example, in publications by Zheleva et al, entitled xe2x80x9cPendeo-Epitaxy: A New Approach for Lateral Growth of Gallium Nitride Filmsxe2x80x9d, Journal of Electronic Materials, Vol. 28, No. 4, pp. L5-L8, February (1999) and Linthicum et al, entitled xe2x80x9cPendeoepitaxy of Gallium Nitride Thin Filmsxe2x80x9d Applied Physics Letters, Vol. 75, No. 2, pp. 196-198, Jul. (1999), the disclosures of which are hereby incorporated herein by reference. Pendeoepitaxy has also been shown to be successful at reducing threading dislocations and cracks caused by lattice mismatch by about three to four orders of magnitude relative to other conventional heteroepitaxy techniques. Nonetheless, because pendeoepitaxy may not always be successful in preventing the formation of cracks and bowing when large compound semiconductor layers such as gallium nitride are cooled to room temperature during back-end processing steps, there continues to be a need for improved methods of forming compound semiconductor layers with reduced susceptibility to cracking and bowing.
It is therefore an object of the present invention to provide improved methods of forming compound semiconductor substrates and substrates formed thereby.
It is another object of the present invention to provide methods of forming compound semiconductor substrates that can be less susceptible to cracking and bowing during back-end processing steps.
These and other objects, advantages and features of the present invention may be provided by methods of forming semiconductor substrates which include the steps of forming a plurality of selective growth regions at spaced locations on a first substrate and then forming a plurality of semiconductor layers at spaced locations on the first substrate by growing a respective semiconductor layer on each of the selective growth regions. The first substrate is then divided into a plurality of second smaller substrates that contain only a respective one of the plurality of semiconductor layers. This dividing step is preferably performed by partitioning (e.g., dicing) the first substrate at the spaces between the selective growth regions. Because this step of dividing the first substrate need not take place until all microelectronic processing steps have taken place, the first substrate with the selective growth regions may be processed on a wafer scale, as with conventional semiconductor wafers.
According to one preferred embodiment of the present invention, the step of forming a plurality of semiconductor layers preferably comprises growing a respective compound semiconductor layer (e.g., gallium nitride layer) on each of the selective growth regions. In particular, the growing step may comprise pendeoepitaxially growing a respective gallium nitride layer on each of the selective growth regions. Each of the selective growth regions is also preferably formed as a respective plurality of trenches that have sidewalls which expose compound semiconductor seeds from which epitaxial growth can take place. In particular, the step of forming a plurality of semiconductor layers may comprises epitaxially growing a first continuous semiconductor layer from sidewalls of a first plurality of trenches (within a first selective growth region) and epitaxially growing a second continuous semiconductor layer from sidewalls of a second plurality of trenches (within a second selective growth region). According to a preferred aspect of this embodiment of the present invention, when the growth steps are completed, the first continuous semiconductor layer and the second continuous semiconductor layer will have opposing edges that are spaced from each other by a wide trench that extends between adjacent edges of the first selective growth region and the second selective growth region.
Advantageously, because each selective growth region is separated from a next adjacent selective growth region by a wide trench the preferably has a width at least about ten (10) times greater than the widths of the narrower trenches within a respective selective growth region, adjacent monocrystalline compound semiconductor layers preferably do not coalesce with each other. Nonetheless, because each monocrystalline compound semiconductor layer is of sufficiently large dimension to ultimately serve as a separate compound semiconductor substrate or xe2x80x9cplatformxe2x80x9d once the supporting substrate has been diced along the dicing streets defined by each wide trench, the failure of the adjacent compound semiconductor layers to coalesce into a single wafer-sized layer need not limit the suitability of the preferred methods for large scale integrated circuit and optoelectronic applications. Instead, by preventing adjacent compound semiconductor layers from coalescing with each other, the likelihood that the compound semiconductor layers will bow or crack upon cooling, as a result of lattice mismatch and thermal expansion coefficient differences, is substantially reduced. In other words, by growing separate compound semiconductor xe2x80x9cplatformsxe2x80x9d separated by wide trenches, the lattice stress and concomitant build-up in strain within each xe2x80x9cplatformxe2x80x9d, caused by differences in the thermal expansion coefficients of the compound semiconductor material and underlying substrate is greatly reduced relative to a large fully coalesced pendeoepitaxially formed compound semiconductor layer.